Mechanism for controlling forces on a strand as it solidifies

ABSTRACT

A method and mechanism for controlling the forces on a partially solidified strand formed in a continuous-casting operation. A speed-regulating tractive force is applied to the strand at a location in the line preceding that where the starter bar is disconnected. In apparatus utilizing a relatively long flexible starter bar, auxiliary tractive forces are applied at preceding locations. In apparatus utilizing a rigid starter bar or a relatively short flexible bar, the auxiliary tractive forces may be applied at locations following the speed-regulating force. The speed-regulating force is maintained at a predetermined maximum, and the auxiliary forces adjusted accordingly to supply the increasing force needed to move the strand, yet avoid excessive tensile or compressive stresses.

United States Patent [191 Gallucci et al.

[ Jan. 21, 1975 1 MECHANISM FOR CONTROLLING FORCES ON A STRAND AS IT SOLIDIFIES [73] Assignee: United States Steel Corporation, Pittsburgh. Pa.

[22] Filed: June 6, 1973 [21] Appl. No.: 367,666

Related US. Application Data [62] Division of Ser. No. 174,350, Aug. 24, 1971, Pat. No,

load signal ONT/70L load adj,

3,566,951 3/ l 971 Schrewe l64/82 Primary ExaminerR. Spencer Annear Attorney, Agent, or FirmWalter P. Wood [57] ABSTRACT A method and mechanism for controlling the forces on a partially solidified strand formed in a continuouscasting operation. A speed-regulating tractive force is applied to the strand at a location in the line preeed ing that where the starter bar is disconnected. In apparatus utilizing a relatively long flexible starter bar, auxiliary tractive forces are applied at preceding locations. In apparatus utilizing a rigid starter bar or a rel atively short flexible bar, the auxiliary traetive forces may be applied at locations following the speed regulating force. The speed-regulating force is maintained at a predetermined maximum, and the auxiliary forces adjusted accordingly to supply the increasing force needed to move the strand, yet avoid excessive tensile or compressive stresses.

6 Claims, 5 Drawing Figures I a I 26%; No. 4

COMPUTER CONTROL /47 E pulse gen. E" for tracking h g CONTROLI I PATENTED 386L456 SHEEI 10F 3 5 0:4 I 1 E: 2 No. I. No.2 No.3 Rage/luring Drive Drive Drive Drive (.l'm Resistance I: .3 h Tola/ Motor Load 2 r Distance from g Hold Barium g 4 m *Force Profile b 5 a 5 Effective E Sim/Id Weigh! 8 PATENTEB M62 1 i975 SHEET 3 OF 3 MECHANISM FOR CONTROLLING FORCES ON A STRAND AS IT SOLIDIFIES This applicatoin is a division of application Ser. No. l74,350, filed Aug. 24, 1971 (now U.S. Pat. No. 3,752,210).

This invention relates to an improved method and mechanism for controlling forces on a continuously cast strand as it solidifies.

In a conventional continuous-casting operation, liquid metal is poured continuously into a vertically oscillating, water cooled, open-ended mold, which may be either straight or curved. A partially solidified strand of indefinite length emerges from the bottom of the mold and travels between series of rolls which engage opposite faces thereof. In the example of a straight mold, these rolls may include a straight vertical guide-roll rack immediately beneath the mold, power driven pinch rolls and/or bending rolls beneath the guide-rollrack; a curved roll-rack which changes the direction of the travel of the strand from vertical to horizontal, a straightener, optionally in-line work-roll stands, and ti nally a run-out table on which the strand is severed to appropriate lengths for further processing. Similar parts may be used with curved molds, except that there are no straight guide-roll-rack nor bending rolls, since the strand is cast with a curvature.

As the strand leaves the mold, only a thin outside skin has solidified, and the core remains liquid. The strand is subjected to intense water sprays as it passes the various rolls, referred to as secondary cooling," whereby it solidifies throughout its cross section by the time it is severed. As long as the core is liquid, forces on the strand must be controlled carefully not to produce defects in the ultimate product. Excessive tensile stresses on a strand may cause it to crack, while excessive compression stresses may cause the strand to bulge. An earlier patent of the present co-inventor Gallucci, U.S. Pat. No. 3,550,674, and Schrewe, Pat. No. 3,566,951 describe some of these problems, but the methods and mechanisms heretofore used for limiting such stresses have not attained precise and positive control we have found desirable.

An object ofour invention is to provide a continuouscasting operation and apparatus in which we employ an improved method and mechanism for controlling lengthwise forces on the solidifying strands, thereby avoiding defects in the ultimate product.

A further object is to provide an improved forcecontrolling method and mechanism in which we apply a speed-regulating tractive force to the strand at a location preceding that where the starter bar is disconnected (e.g., at the straightener), maintain the speedregulating force at a predetermined maximum, and apply controlled auxiliary tractive forces at other locations to minimize or eliminate stresses in the strand.

A further object, applicable to apparatus utilizing a relatively long flexible starter bar, is to provide a forcecontrolling method and mechanism in which we subject the strand to a speed-regulating drive at an advanced location in the line, and to a series of adjustable auxiliary tractive forces along its length between the mold and the speed-regulating drive, whereby we maintain lengthwise forces in the strand at any desired value or eliminate tension altogether in favor of a compression stress.

In the drawing:

FIG. I is a partly diagrammatic vertical section of a continuous-casting apparatus which utilizes a relatively long flexible starter bar and is equipped with one embodiment of our force-controlling mechanism, the parts being shown in the position they occupy shortly after the start of a casting operation;

FIG. 2 is a graph showing a typical relation of the forces on a strand in an apparatus such as that shown in FIG. 1;

FIG. 3 is a block diagram of an electric circuit suitable for use in the embodiment of our mechanism shown in FIG. 1;

FIG. 4 is a block diagram of a similar circuit, but embodying computer control for automatic operation; and

FIG. 5 is a partly diagrammatic vertical section of a continuous-casting apparatus which utilizes a rigid starter bar and is equipped with a modified embodiment of our force-controlling mechanism, the parts being shown in the position they occupy after the casting operation is underway and the starter bar disconnected.

The continuous-casting apparatus illustrated in FIG. 1 includes from top to bottom a straight mold 10, a straight vertical guide-roll-rack 12, a bending roll unit 13, a curved roll-rack 14, a straightener 15, and a runout table 16. Liquid metal is introduced to the mold from a tundish 17 supported thereabove. At the beginning of a casting operation, a starter bar 18 is inserted in the lower end of the mold and extends through and beyond the straightener 15. A partially solidified strand S emerges from the lower end of the mold and descends following the starter bar between the various sets of rolls. After the leading end of the strand S clears the straightener 15, the starter bar is disconnected and stored in readiness to start the next cast.

Numerous variations in the structure of the casting apparatus of course are possible. The apparatus illus trated in FIG. 1 utilizes a relatively long flexible starter bar, which is not disconnected until it reaches the runout table, but our invention in modified form can be used with apparatus which utilizes a starter bar (rigid or flexible) disconnected above the bending rolls, as shown in FIG. 5 and hereinafter described. Also it can be used with apparatus which has a curved mold, or which has a varying radius of curvature in the curved roll-rack, or which has in-line work rolls, etc., all of which are well known in the continuous-casting art.

In accordance with the embodiment of our invention illustrated in FIG. I, the apparatus has sets of driven rolls 20 and 200 which engage opposite faces of first the starter bar 18 and later the strand S at an advanced location in the line and which we refer to as our "speed-regulating drive". This embodiment has a first set of auxiliary driven rolls 21, which engage opposite faces of the strand near the bottom of the mold and which we refer to as our No. 1 drive. Between our No. I drive and our speedregulating drive, the apparatus has longitudinally spaced sets of auxiliary driven rolls 22 and 23, which we refer to as our No. 2 drive and No. 3 drive" respectively. All the other rolls shown can be idlers. In the apparatus illustrated, the rolls 20 and 20a of our speed-regulating drive are located fore and aft of the straightener 15; our No. 1 drive 21 is located between the guide-roll rack 12 and the bending rolls [3; our No. 2 and 3 drives 22 and 23 are located intermediate the length of the curved rollrack 14. Nevertheless it is apparent that the exact location and number of these drives can vary.

At our No. 1 drive 21 the strand S has only a thin skin which is easily ruptured. Hence the pressure which each roll of this drive exerts against the strand must be relatively low. In order to produce the necessary tractive force on the strand, as hereinafter explained, without exerting excessive pressure at any one location thereon, we use a plurality of closely adjacent pairs of rolls at this drive (four pairs in the illustration). At our No. 2 and 3 drives 22 and 23, the strand still has a liquid core, but its skin is thicker. For the same reason, we prefer to use a plurality of pairs of rolls also at each of these drives, but the number can be smaller (two pairs each in the illustration). Likewise we use two pairs of rolls in the speed-regulating drive, but place them on opposite sides of the straigntener, as already described.

At the beginning of a casting operation, when the starter bar 18 extends between the various sets of rolls, our speed-regulating drive 20, 20a acts as a brake to restrain the starter bar from descending under its own weight. Initially we operate our auxiliary drives with their current set points at zero, whereby they act as idlers. As the casting operation proceeds, the leading end of the strand S passes through the guide roll-rack I2, our No. I drive 21 (operating as an idler at this time), and into the bending roll unit 13. As soon as the strand commences to bend, some tractive force is needed to propel it. At first the speed-regulating drive 20, 20a furnishes this force. The farther the strand progresses, the greater the resistance to its movement and the greater the tractive force or torque needed to propel it at a given speed.

Conveniently the motors for the various drives can be constant-field d-c motors, which have a characteristic that at any speed their torque output is directly proportional to the current value. When the current to the speed-regulating drive reaches or commences to exceed a predetermined maximum (for example amperes), we increase the current to our No. 1 drive to supply the increasing tractive force needed to propel the strand, and thereafter we maintain the current to the speed-regulating drive at this maximum. When the leading end of the strand successively passes through our No. 2 and 3 drives 22 and 23, we increase the current to each of these drives in turn to supply the increasing tractive force, thereafter keeping the current constant to the speed-regulating drive and to No. 1 drive, and subsequently to No. 2 drive after No. 3 drive takes over. After the casting operation is fully underway and the strand extends all the way from the mold 10 to the run-out table 16, the current to the speedregulating drive 20, 200 may remain at the exemplary maximum of IO amperes, and the current to the No. l, 2 and 3 drives 2|, 22 and 23 may be at 20, and I0 amperes respectively.

which must be supplied to move the strand. Curve C also represents the magnitude of tensile stresses in the strand at various points along its length if the speedregulating drive 20 alone were used to pull the strand through the apparatus. Curve D shows an example of the magnitude of tensile stresses in the strand when our method is followed. From the mold 10 to our No. I drive, Curve D is the same as Curve C. At No. I drive we apply a tractive force to the strand and thereby place it in compression. Following No. I drive Curve D rises parallel with Curve C until it reaches No. 2 drive. where we again apply a tractive force which restores the compression. The same relation takes place between No. 2 drive and No. 3 drive, and again between No. 3 drive and the speed-regulating drive. In this manner the strand remains under slight compression substantially throughout the operation.

FIG. 3 illustrates in block diagram a typical electric circuit we can use for manually controlling the various drive motors. Constant-field dc motors for the two sets of rolls 20 and 20a of the speed-regulating drive are indicated at 26 and 260 respectively. We energize these motors through a closed-loop speed-regulating circuit 28, which includes a power supply 29, an amplifier 30 and a tachometer 31. We control the speed of motors 26 and 26a through a master pilot 32, which we can adjust either manually or through a liquid-level control mechanism on the mold, as shown for example in Tiskus et al Patent No. 3,300,820. The two motors 26 and 26a run at essentially a constant speed, which we change only to change the strand speed. An ammeter 33 indicates the magnitude of current the motors draw to maintain this speed, which magnitude is a measure of the torque output of the motors.

When a casting operation is getting underway, the magnitude of current which the motors 26 and 26a draw continually increases, since the line resistance to movement of the strand increases, as shown by Curves C and D of FIG. 2. When this current reaches a predetermined magnitude (for example l0 amperes) as indicated by ammeter 33, we increase the current to our No. I drive 2], which has a constant-field d-c motor 36 energized through a closed-loop current-regulating circuit 37. The latter includes a power supply 38, an amplifier 39, a manual load adjustment 40 and an ammeter 41. We connect the master pilot 32 to the amplifier 39, whereby motor 36 rotates the rolls of No. I drive at the same peripheral speed as the rolls of the speedregulating drive 20, 200. We gradually increase the current to motor 36 as needed to maintain the current to motors 26 and 26a at the predetermined magnitude as a maximum. Thus No. I drive supplies the increasing tractive force needed to move the strand. At this time the leading end of the strand S has passed No. I drive and is approaching No. 2 drive 22.

Our No. 2 and 3 drives 22 and 23 have closed-loop current-regulating circuits 44 and 45 which are similar to circuit 37 of No. I drive. After the leading end of the strand reaches No. 2 drive, we maintain the current to both motors 26 and 26a of the speed-regulating drive and to motor 36 of our No. I drive at constant magnitudes and increase the current to No. 2 drive to supply the increasing tractive force needed to move the strand. After the leading end reaches No. 3 drive, we use this drive in the same fashion, keeping the current to No. 2 drive constant.

HO. 4 illustrates in block diagram the way in which we can make the foregoing adjustments automatically. We connect a pulse generator 46 to motor 260 of our speed-regulating drive to track the leading end of the strand. We connect the pulse generator 46 to a digital computer 47, which we in turn connect to the control circuits 37, 44 and 45 for No. l, 2 and 3 drives. In this manner the computer automatically adjusts the current to each drive to achieve the same effect as in the manual control already described.

FIG. 5 illustrates a continuous-casting apparatus in which the starter bar 50 is rigid and to which we may apply our invention in modified form. The apparatus includes from top to bottom a straight mold 51, a straight vertical guide-roll-rack 52, power-driven pinch rolls 53, a curved roll-rack 54, a straightener 55, and a horizontal roll-rack 56. The curved roll-rack has at least one switch section 57 in which some of the rolls are journaled. The switch section can open to allow the starter bar 50 to descend vertically after it is disconnnected from the leading end of the strand, as shown for example in Foldessy Pat. No. 3,338,297.

When we apply our invention to an apparatus constructed as shown in FIG. 5, the pinch rolls 53 become the speed-regulating drive. We use a plurality of closely adjacent pairs of pinch rolls to spread the force along the strand, similar to the No. 1 drive of FIG. 1. At the beginning of a casting operation, the pinch rolls 53 engage the starter bar and act as a brake to restrain it from descending under its own weight. After the starter bar is disconnected and the switch section 57 closed, the strand S enters the curved roll-rack S4 and requires an increasing tractive force to propel it. At spaced locations along the curved roll-rack, we provide auxiliary pairs of driven rolls S8, 59 and 60 which we again refer to as our No. l, 2 and 3 drives. The rolls in the straightener 55 are driven, but all the other rolls can be idlers.

At the beginning we operate the auxiliary drives 58, 59 and 60 as idlers, the same as in the apparatus shown in FIG. 1. When the leading end of the strand passes through No. I drive 58, we increase the current to this drive to supply the increasing tractive force needed to propel the strand, and thereafter maintain the current to the speed-regulating drive (pinch rolls 53) constant. When the leading end of the strand successively passes through our No. 2 and 3 drives 59 and 60 and finally the straightener 55, we increase the current to each, the same as in FIG. I, keeping the current to the preceding drives constant.

From the foregoing description, it is seen that our invention assures that a solidifying continuously-cast strand is relieved of any excessive longitudinal stresses as it solidifies. Preferably we maintain a small compressive stress during most of the operation, as Curve D of H6. 2 indicates. in the apparatus shown in FIG. 1, the speed-regulating drive pulls the strand, and our auxiliary drives act to relieve excessive compressive stresses which tend to bulge the strand. in this manner our invention avoids defects in the strand caused by lengthwise stresses, and permits a higher casting speed for crack-sensitive steel grades.

We claim:

1. In a continuous-casting apparatus which includes a mold from the bottom of which a partially solidified strand of indefinite length may descend continuously, series of rolls below said mold for engaging opposite faces of the strand as it travels therebetween, at least some of said rolls defining a curved path in which the strand bends, whereby its direction of travel becomes substantially horizontal, and a starter bar to be connected to the leading end of said strand as it emerges from the mold and disconnected therefrom at a location spaced below the mold; the combination therewith of an improved mechanism for controlling forces on the strand, said mechanism comprising:

means operatively connected with selected rolls of said series for applying through said selected rolls first a force restraining descent of said starter bar and a strand connected thereto, and subsequently a speed-regulating tractive force as the leading end of the strand advances through said curved path. whereby a tractive force becomes needed to propel the strand, said selected rolls being at a location preceding the location at which said starter bar is disconnected;

a plurality of auxiliary drive means operatively connected with other selected rolls of said series at locations spaced apart from each other and from said first-named selected rolls, said other selected rolls being operable as idlers when said auxiliary drive means are deenergized;

control means operatively connected with the first auxiliary drive means nearest the mold for energizing this drive means when the speed-regulating tractive force reaches a predetermined maximum and thereby applying a first auxiliary tractive force to the strand; and

additional control means operatively connected with each other auxiliary drive means for energizing the other auxiliary drive means in sequence when the force on the preceding other auxiliary drive means reaches a predetermined maximum, and thereby applying additional auxiliary tractive force to the strand;

said speed-regulating tractive force and said auxiliary tractive forces minimizing lengthwise stresses in the strand throughout a casting operation.

2. A mechanism as defined in claim 1 in which said first-named selected rolls engage the strand at an advanced location with respect to said mold and pull the strand through said series of rolls, and said other selected rolls engage the strand at locations between said mold and said first-named selected rolls to relieve ten sile stresses in the strand.

3. A mechanism as defined in claim 1 in which said first-named selected rolls engage the strand at a location near said mold and push the strand through said series of rolls, and said other selected rolls engage the strand at locations following said first-named selected rolls to relieve compressive stresses in the strand.

4. A mechanism as defined in claim I in which said series of rolls include in succession from said mold downward a guide roll-rack, binding rolls, a curved roll-rack and a straightener. said starter bar is flexible. said first-named selected rolls are in said straightener and pull the strand through said series of rolls, and said other selected rolls engage the strand at locations between said mold and said straightener to relieve tensile stresses in the strand.

5. An apparatus as defined in claim I in which said series of rolls include in succession from said mold downward a guide roll-rack, power driven pinch rolls, bending rolls, a curved roll-rack, and a straightener. said starter bar is rigid, said pinch rolls are said firstfirst-named selected rolls to receive load signals therefrom and to said auxiliary drive means to energize the latter in response to load signals received from said first-named selected rolls.

l i i 

1. In a continuous-casting apparatus which includes a mold from the bottom of which a partially solidified strand of indefinite length may descend continuously, series of rolls below said mold for engaging opposite faces of the strand as it travels therebetween, at least some of said rolls defining a curved path in which the strand bends, whereby its direction of travel becomes substantially horizontal, and a starter bar to be connected to the leading end of said strand as it emerges from the mold and disconnected therefrom at a location spaced below the mold; the combination therewith of an improved mechanism for controlling forces on the strand, said mechanism comprising: means operatively connected with selected rolls of said series for applying through said selected rolls first a force restraining descent of said starter bar and a strand connected thereto, and subsequently a speed-regulating tractive force as the leading end of the strand advances through said curved path, whereby a tractive force becomes needed to propel the strand, said selected rolls being at a location preceding the location at which said starter bar is disconnected; a plurality of auxiliary drive Means operatively connected with other selected rolls of said series at locations spaced apart from each other and from said first-named selected rolls, said other selected rolls being operable as idlers when said auxiliary drive means are deenergized; control means operatively connected with the first auxiliary drive means nearest the mold for energizing this drive means when the speed-regulating tractive force reaches a predetermined maximum and thereby applying a first auxiliary tractive force to the strand; and additional control means operatively connected with each other auxiliary drive means for energizing the other auxiliary drive means in sequence when the force on the preceding other auxiliary drive means reaches a predetermined maximum, and thereby applying additional auxiliary tractive force to the strand; said speed-regulating tractive force and said auxiliary tractive forces minimizing lengthwise stresses in the strand throughout a casting operation.
 2. A mechanism as defined in claim 1 in which said first-named selected rolls engage the strand at an advanced location with respect to said mold and pull the strand through said series of rolls, and said other selected rolls engage the strand at locations between said mold and said first-named selected rolls to relieve tensile stresses in the strand.
 3. A mechanism as defined in claim 1 in which said first-named selected rolls engage the strand at a location near said mold and push the strand through said series of rolls, and said other selected rolls engage the strand at locations following said first-named selected rolls to relieve compressive stresses in the strand.
 4. A mechanism as defined in claim 1 in which said series of rolls include in succession from said mold downward a guide roll-rack, binding rolls, a curved roll-rack and a straightener, said starter bar is flexible, said first-named selected rolls are in said straightener and pull the strand through said series of rolls, and said other selected rolls engage the strand at locations between said mold and said straightener to relieve tensile stresses in the strand.
 5. An apparatus as defined in claim 1 in which said series of rolls include in succession from said mold downward a guide roll-rack, power driven pinch rolls, bending rolls, a curved roll-rack, and a straightener, said starter bar is rigid, said pinch rolls are said first-named selected rolls and push the strand through the series of rolls which follow, and said other selected rolls engage the strand following said pinch rolls to relieve compressive stresses in the strand.
 6. An apparatus as defined in claim 1 in which said control means includes a computer connected with said first-named selected rolls to receive load signals therefrom and to said auxiliary drive means to energize the latter in response to load signals received from said first-named selected rolls. 